Clamping devices for tool spindles used in machine tools serve to clamp a tool. They normally comprise a tubular sleeve and a draw or tie rod axially movable to a limited extent in the sleeve. The sleeve has an internal cone, widening toward the distal end thereof, which is matched to a corresponding cone on the tools to be used with the spindle (e.g., standardized ISO or HSK interfaces). For attaching a tool to the spindle, the tool is coupled with the aid of a tool holder to the draw rod and the draw rod is drawn with a tensile force towards the proximal end of the sleeve. As a result, the tool cone is drawn into the internal cone of the sleeve and pressed therein. The frictional grip between the tool cone and the internal cone strengthens the spindle/tool unit and substantially absorbs torque acting on the tool during operation.
The sleeve is rotatably mounted in the spindle casing and, e.g., directly carries the rotor of an electric motor which drives the clamping device and a tool fixed thereto.
The drawing-in force necessary for the attachment of the tool in the clamping device is normally produced by a spring assembly or bank, which is positioned under pretension between the sleeve and draw rod. For removing the tool, the draw rod is moved counter to the spring tension of the spring assembly toward the distal sleeve end with a usually hydraulically operated piston acting on the proximal end of the draw rod. As a result, the tool cone is slid out of the internal cone of the sleeve and the connection between tool and draw rod is released, so that the tool can be removed.
As stated, the torque which can act during a working process on the tool of a machine tool is absorbed by the connection between clamping device and tool and is upwardly limited, and in a connection by cone/internal cone in particular, by the tensile force exerted on the draw rod. For higher power levels and higher speeds normally higher torques occur, so that in many cases it would be desirable to be able to increase the tensile force. As the tensile force also influences the static and dynamic stiffness of the unit constituted by the tool and the clamping device, and consequently also influences the attainable machining precision, it would also be desirable in this connection to be able to raise the tensile force.
The tensile force is primarily dependent on the spring characteristic (spring constant) of the spring assembly acting between sleeve and draw rod, which normally comprises a plurality of disk springs. It has been found that it is not easy to modify the spring characteristic of this spring assembly for significantly increasing the tensile force. This is due to the substantially predetermined geometrical conditions, because the external diameter of the sleeve is predetermined by the mounting and optionally by a motor, whose rotor it forms.